1. Field of the Invention
The present invention relates to a gas-fueled internal combustion engine operable with gaseous fuel such as hydrogen or the like.
2. Background of the Invention
As described in Japanese Patent Application Publication No. JP-A-6-88542 for example, hydrogen, a gaseous fuel, may be used as a fuel for an internal combustion engine. Hydrogen has a combustibility range as broad as 4-75 percent by volume and can be readily burned in an extremely lean air-fuel mixture having an air excess ratio λ that is equal to or greater than about 4. Thus, if hydrogen is used as a fuel for an internal combustion engine, power can be extracted even at an extremely lean air-fuel ratio, thereby making it possible to realize what is called a “super lean burn operation”.
In a super lean burn operation, it is possible to open a throttle valve fully, thereby reducing pumping loss and decreasing combustion temperature, which in turn leads to a reduction in a cooling loss. Such reduction in the pumping loss and the cooling loss improves engine efficiency so that the engine can be operated with an excellent fuel economy and high efficiency. Furthermore, the reduction in the combustion temperature helps to suppress NOx emission and the use of hydrogen fuel can also prevent any production of CO2 and CO. This means that the hydrogen-fueled super-lean burn operation can realize a completely zero emission.
However, it is difficult to realize the hydrogen-fueled super lean burn operation in a high load region since while the quantity of hydrogen fuel has to be increased in the high load region in order to generate a required output power to realize the operation, there is a limit on the quantity of intake air that can be charged for that purpose. For this reason, as compared with the amount of hydrogen, that of the intake air runs short in the high load region, thereby making it impossible to maintain a super lean burn air-fuel ratio needed.
An air-fuel ratio heavily influences the efficiency and emission control in an internal combustion engine. FIG. 32 illustrates the influence of an air excess ratio λ on the thermal efficiency and the cooling loss of an internal combustion engine wherein the X-axis represents the air excess ratio λ and the Y-axis refers to the thermal efficiency and the cooling loss. As illustrated in this graph, the cooling loss increases as the air excess ratio λ decreases. In particular, the cooling loss increases sharply if the air excess ratio λ becomes smaller than 2. As a consequence, the thermal efficiency of an internal combustion engine reaches a peak point when the air excess ratio λ is around 2, and then gradually decreases as the air excess ratio λ decreases in the area where λ is smaller than 2.
FIG. 33 represents graphs corresponding to the variation of the NOx emission amount with respect to the air excess ratio λ in cases where hydrogen and gasoline are used as fuel, respectively, wherein the X-axis denotes the air excess ratio λ and the Y-axis refers to the NOx emission amount. As can be seen in these graphs, in the case where hydrogen is used as fuel, the NOx emission amount is nearly zero when the air excess ratio λ is greater than 2. However, when the air excess ratio λ is smaller than 2, the NOx emission amount in the case of hydrogen being used as the fuel drastically increases to where it is greater than the NOx emission amount in a case where gasoline is used as the fuel.
As noted above, the hydrogen-fueled internal combustion engine (“hydrogen internal combustion engine”) is essentially able to operate with high efficiency and low emission as long as the air excess ratio λ is kept greater than 2, but suffers from reduced efficiency and a sharp increase in the NOx emission amount when the air excess ratio λ is smaller than 2. This is because the combustion rate of hydrogen is several times higher than (a little less than ten times) that of a hydrocarbon fuel such as gasoline or the like and further because the combustion proceeds even more rapidly as the air-fuel ratio comes closer to a stoichiometric air-fuel ratio (namely, λ=1). FIG. 34 depicts graphs corresponding to the variation of a heat generation rate with respect to a crank angle at λ=1 and λ=2, respectively, wherein the X-axis denotes the crank angle and the Y-axis refers to the heat generation rate. As shown in these graphs, the heat generation rate is gently increased and has a lower peak at λ=2. On the other hand, at λ=1, the heat generation rate undergoes a sharp increase and has a higher peak. The combustion of an air-fuel mixture in a combustion chamber occurs rapidly and furiously as the peak of the heat generation rate becomes higher.
Such rapid combustion leads to an increased combustion temperature within the combustion chamber. Consequently, the cooling loss increases and the NOx emission amount soars drastically as set forth above. Moreover, in practice, the pressure-increase rate within a cylinder becomes significantly high, so that a flame noise and an engine body damage noticeably increase. These factors make it difficult for conventional hydrogen internal combustion engine to operate under sustained high loads.